Difference between revisions of "NVGate Octave Analyzer"

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NVGate Octave Analyzer (view source)

Revision as of 09:54, 30 April 2020

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→‎NVgate 1/n octave plug in
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== NVgate 1/n octave plug in==
==Structure==
 
<font size = "5">'''8.2. REAL TIME 1/N'''</font><font size = "6"><sup>'''th</sup></font><font size = "5"> ''''''OCTAVE ANALYZER'''</font>
 
----
 
<font size = "4">'''8.2.1. Structure and Operation of the Real-Time Mode'''</font>
 
The figure below shows the block diagram of the analyzer (for one channel):
 
After amplification and antialias filtering, analog input signals are sampled at 51.2 kHz (or 25.6 kHz or 12.8 kHz or 5.12 kHz) and converted by an analog to digital converter.
 
The sample frequency depends on the value of the High Filter Setting (see 8.2.14.3 Settings <nowiki>|</nowiki> Analysis - High Filter)
 
Next, the input signals can be time weighted by three types of filters at user choice:
 
<u>Time domain integrator filters:</u>
 
Single or double time integrators are based on third order high pass filters and the low frequency cut off is automatically adjusted to frequency range.
 
The error on frequency response for a single integrator is less than 0.02 dB for frequencies less than 4 kHz and less than 0.15 dB for frequencies greater than 4 kHz.
 
For more information, see <font color="#FF0000">'''8.2.7.7''' </font>'''Settings <nowiki>|</nowiki> Preprocess - Digital Time Integrator'''.
 
<u>A, B and C weight filters</u>
 
These filters available in acoustic frequency range (i.e. from 20 Hz to 20 kHz) satisfy requirements from standards IEC 651 type 0 and IEC 804 type 0.
 
 
----
 
{|border="0" cellspacing="2" width="100%"
|'''8-8'''
|'''F 197 804 1'''
 
|}
 
----
 
<u>Spectral Weight Filters:</u>
 
These filters are only available with the OR7937 option.
 
For more information, see <font color="#FF0000">'''8.2.7.5''' </font>'''Settings <nowiki>|</nowiki> Preprocess - Spectral Weight Filter'''.
 
After time domain filtering, the signals go to the digital 1/n<sup>th</sup> octave filter bank.
 
This filter bank is based on the 6<sup>th</sup> order digital band pass. A downsampling for low central frequency filters reduces computation load. The downsampling filters have a rejection greater than 90 dB at half of each new sampling rate and a ripple less than 0.007 dB in the useful bandwidth.
 
The filter range uses base 10 so that we get exact frequencies at 0.1 Hz, 1.0 Hz, 10 Hz, 100 Hz, 1 kHz and 10 kHz.
 
The following calculations are used to compute the central frequencies:
 
{|border="2" cellspacing="0" cellpadding="4" width="40%"
|•
|fc = 1000
|<font size = "5"><sub><nowiki>*</nowiki> 103n/10</sub></font><br>
|for 1/1 octave
 
|-
|•
|fc = 1000
|<font size = "5"><sub><nowiki>*</nowiki> 10n/10</sub></font><br>
|for 1/3<sup>rd</sup> octave
 
|-
|•
|fc = 1000
|<font size = "5"><sub><nowiki>*</nowiki> 10(n<nowiki>+</nowiki>0.5)/40</sub></font><br>
|for 1/12<sup>th</sup> octave
 
|-
|•
|fc = 1000
|<font size = "5"><sub><nowiki>*</nowiki> 10(n<nowiki>+</nowiki>0.5)/80</sub></font><br>
|for 1/24<sup>th</sup> octave
 
|}
 
Number of filter and frequency range for each filter bank:
 
* 1/1 octave filter bank:
 
* can have up to 11 filters (with 1 to 1000 frequency ratio),
 
* covers range from 125 mHz to 16 kHz.
 
* 1/3<sup>rd</sup> octave filter bank:
 
* can have up to 31 filters (with 1 to 1000 frequency ratio),
 
* covers range from 100 mHz to 20 kHz.
 
* For 1/12<sup>th</sup> octave:
 
* the filter bank gets 4 filters for each useful bandwidth of 1/3<sup>rd</sup> filters, i.e. 124 filters,
 
* covers range from 92 mHz to 21.8 kHz.
 
* For 1/24<sup>th</sup> octave:
 
* the filter bank gets 8 filters for each useful bandwidth of 1/3<sup>rd</sup> filter, i.e. 248 filters,
 
* covers range from 90 mHz to 22.1 kHz.
 
Filters for 1/1 and 1/3<sup>rd</sup> octave analysis conform to the IEC 225 standard.
 
The OR7933 analyzer satisfies requirements from standard ANSI S1.11 - 1986 Order 3 Type 1D optional and extended range, based on maximum flat Butterworth filters. Filters bandwidth is adjusted, as recommended in ANSI 1.11, for minimum white noise bandwidth error.
 
For more information on filter bank setup, see <font color="#FF0000">'''8.2.14.3''' </font>'''Settings <nowiki>|</nowiki> Analysis - High Filter''' and <font color="#FF0000">'''8.2.14.4''' </font>'''Settings <nowiki>|</nowiki> Analysis - Low Filter '''chapters.
 
----
 
----
 
----
 
 
The next step is the detector process for each 1/n<sup>th</sup> filter:
 
[[Image:oct_01.png|framed|none]]
 
<font color="#FF0000">''Figure 8-2 ''</font>''Detector''
 
The filtered signal is input to a squaring module in order to get true RMS detector.
 
This module computes 1/N * Σx²<sub>n.</sub>
 
The instant averaging is always running and is based on the exponential averaging with a time constant equal to 1/fc where fc is the center frequency of corresponding filter. So each detector has its own time constant and the output fluctuation in the worst case is limited to <nowiki>+</nowiki>/- 0.4 dB with a sine input signal.
 
The averaging process provides a lot of modes (linear, exponential and dedicated to acoustical measurements) which are described in the <font color="#FF0000">'''8.2.14.5''' </font>'''Settings <nowiki>|</nowiki> Analysis - Average Mode''' chapter.
 
A hold box allows to get Maximum and Minimum spectra during one measure.
 
A stabilization delay is implemented in order to ignore and suppress the transient response of passband filters. It is automatically taken into account after any change of input setup. It is equal to 5 periods of the lower frequency filter for 1/3<sup>rd</sup> octave and octave filters. This delay is four times greater for 1/12<sup>th</sup> octave filters (i.e. 20 periods of the lower frequency filter) and eight times greater for 1/24<sup>th</sup> octave filters (i.e. 40 periods of the lower frequency filter).
 
For example, if lower frequency filter is centered at 1 Hz, then the stabilization delay is equal to 5 seconds for octave and 1/3<sup>rd</sup> octave, 20 seconds for 1/12<sup>th</sup> octave and 40 seconds for 1/24<sup>th</sup> octave. During this delay the detectors are inactive.
 
Finally, the analyzer stores spectrum results in order to obtain waterfall spectrum. However, the 1/n<sup>th</sup> Waterfall is somewhat different from other modes (as FFT or Tracking) in terms of implementation:
 
The filter bank continuously runs and instant averaging is always available (excluding time period for filter stabilization).
 
In linear modes, trigger events are used to start and/or stop energy computation in detectors and store associated results in the spectrum waterfall memory.
 
In exponential modes, detectors are always running and trigger events are only used to store associated results in the spectrum waterfall memory.
 
----
 
----
 
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The other modules used are:
 
* An arming module that is used after a START command to arm trigger for the first event detection.
 
* A trigger module based on free start, manual, input level, delta time, tachometer with ∆ RPM capabilities and input with ∆ V variation.
 
For more information about arming and triggering, see corresponding chapters in Real Time FFT mode:
 
<font color="#FF0000">'''4.2.8 '''</font>'''RPM Tach Setup'''
 
<font color="#FF0000">'''4.2.11 '''</font>'''Arming Setup'''
 
<font color="#FF0000">'''4.2.12 '''</font>'''Triggering Setup'''
 
<font color="#FF0000">'''4.2.13 '''</font>'''Input Level Setup'''
 
<font color="#FF0000">'''4.2.14 '''</font>'''Delta Level Setup'''
 
An internal generator is also available (see <font color="#FF0000">'''4.2.19''' </font>'''Generator Setup''' in the real time FFT mode).
 
The PC ensures the real-time display functions of the traces, the management of the graphic zooms, the decoding and execution of the commands linked to the menus.
 
The '''OR7933''' analyzer can manage up to 16 channels that can be selected among up to 16 inputs. A special dialog box (see <font color="#FF0000">'''8.2.7''' </font>'''Input Preprocess Setup''' chapter in the real time mode chapter) is used to link the input channels to a preprocess on the analyzer hardware.
 
 
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{|border="0" cellspacing="2" width="100%"
|'''F 197 804 1'''
|'''8-11'''
 
|}
 
----
 
'''Nth OCTAVE ANALYZER'''
 
'''REAL TIME 1/Nth OCTAVE ANALYZER'''
 
----
 
{|border="0" cellspacing="2" width="100%"
|'''8-50'''
|'''F 197 804 1'''
 
 
== NVGate 1/n octave plug in==
This plug-in computes 1/1, 1/3, 1/12 and 1/24 octave analysis based on order 3 digital filters and RMS detectors.
This plug-in computes 1/1, 1/3, 1/12 and 1/24 octave analysis based on order 3 digital filters and RMS detectors.


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